In modern biochemical analytics, immunoassays are routinely used to detect the presence or concentration of various substances, often referred to as ligands or analytes, in biological fluids such as blood, urine or saliva. In a solid phase immunoassay, a binding agent, typically an antibody which is specific for the ligand to be detected, is immobilized on a solid support. A test fluid that may comprise the ligand to be detected is contacted with the solid support and a complex between the binding agent and the ligand is formed in case the ligand is present. In order to make the complex visible, labeled antibodies may be used that bind to the complex followed by visual detection of the labeled antibody bound to the complex.
U.S. Pat. No. 5,591,645 discloses a sandwich immunoassay, wherein a ligand is sandwiched between a labeled antibody and an antibody immobilized on a solid support.
Porous materials such as nitrocellulose, nylon, cellulose acetate, glass fibers and other porous polymers have been employed as solid supports in solid phase immunoassays. In so-called lateral flow assays, a fluid to be tested for the presence of a ligand is applied to one end of a porous membrane layer and flows in lateral direction through the membrane under the action of capillary forces. The porous membrane comprises an immobilized binding agent that is capable of binding the ligand to be detected. The immobilized binding agent may be evenly distributed over the entire membrane. Typically, however, the immobilized binding agent is located in defined test or detection zones in the membrane, usually in narrow test lines that have been applied by means of inkjet printing or other aerosol spraying techniques.
In lateral flow immunoassays, typically a thin layer of microporous material with immobilized binding agent is supported on a liquid-impermeable layer to provide sufficient rigidity to the fragile membrane layer. Usually a layer of microporous material with a thickness in the range of from about 100 to 200 μm is supported on a support layer, usually referred to as “membrane backing”. However, depending on the requirements, the membrane thickness may be thinner than 100 or thicker than 200 μm.
Frequently occurring problems with lateral flow immunoassays relate to assay sensitivity and, associated therewith, signal intensity. Such assays are typically five to ten times less sensitive than for example an Enzyme-Linked Immuno Sorbent Assay (ELISA). Several measures to enhance the signal have been proposed, for example signal amplification strategies such as enzymatic enhancement of the signal, but there is still room for improvement.
Thus, the object underlying the present invention is to provide a lateral flow membrane arrangement resulting in an improved assay sensitivity and signal intensity.